Two-way transmission with repeaters



2 Sheets-Sheet l E. D. JOHNSON Filed Feb. '7, 1923 TWO-WAY TRANSMISSION WITH REPEA'IERS //7 venfor: 0gc7r/0 J0/7/7a0/7. y A 1 Patented Feb. 17, 1925.

[UNITED STATES PATENT OFFICE.

EDGAR D. .ionNsoN, 0F EAsT ORANGE, NEW JERSEY, AssioNoE 'ro WESTERN ELEC- TRIe coMrA Y, INCORPORATED, on NE YORK, N. Y., A CORPORATION or NEW "YORK.

. TWO-WAY TRANSMISSION WITH RiEPEATERS.

Application filed February. 7, 1923. Serial No. 617,512.

To all it may concern: Be it. known that I, EDQAR D. JOHNSON, a citizen of the United States, residingat East Orange, in the countyof Essex, State" of New. Jersey, have invented certain new and hseful, Improvements in Two-Way Transmissions With Repeaters, of whichthe following is a full, clear, concise, and exact description. 3 v

This invention relates to repeater circuits, and more particularly to telephone lines. in

' which are employed one or more repeaters for carryingcurrents in one direction and one or more other repeaters for carrying currents in the opposite direction.

The object of the invention is to improve "transmission in such circuits, andespecially to reduce the transmission in a given ,dircction of currents transmitted inthe opposite direction, to thereby reduce echo elfects .in'the system or prevent singing of the repeaters, or both, as explained hereinafter. The nature and effects ofecho currents have been explained by A. B. Clark in his article on Telephone transmissionover.long cables, Journal of the American Institute of Electrical Engineers, Jan. 1923, page 1, and referencemaybe had to that article for abetterunderstanding of the phenomena of echo effects. i r In. accordance with the invention two lines either with or without balancing networks are connected by ,atwo-way repeater circuit haying the amplification of the repeatersy. when transmission is not taking place, adgusted to alues too low to cause singing; and when normal transmission takes place through the repeatingpath in either direction a relay connected to that path and responsive to voice currentscauses ,a, decrease in the transmitting etliciency'of the repeatingpath in the opposite direction (in the general fashion disclosed in -Mills Patent No. 1,434,790, NOVell1b6l'..7, "1922, two-way transmission with repeaters), and may also cause an increase in the transmitting eflicieney of the repeating path through which the desired transmission is .to pass. Eaeh voice relay is so connected to the repeating path with which it is asso' flciated, that when the transmitting efiiciency of the path is decreased the relay is rendered ineffective to cause the transmitting cilicieney of the other path to be decreased.

Fig. 1 of the drawing is a circuit diagram of a form of the invention as applied to a four-wire repeating system; Fig. 2 shows, somewhat in detail, voice relay operating circuits which in Fig. 1 are merely blocked in; and Figs. 3 and 1 show modifications of the'circuit of Fig. 1, the circuit of Fig. 41 employing no networks for balancing the impedances of the lines between which the repeating paths are connected.

By a four-wire line or circuit is meant one inwhich the talking currents in one direction for the greater portion of the distance between two points geographically remote from each other are carried overa difi'erent pair of wires from the currents in the other direction, as distinguished from a two-wire circuit which is one in which thetalking currents in both directions between two points are carried over the same pair of wires for the greater portion of the distance betweenthose points. 7

The four-wire repeating system shown in Fig. .1 isof the general type disclosed in Campbell Patent No. 1,352,786, September 14, 1920, .four-wiro transmission system, lines 1 and 2 being separate transmission lines forming a four wire transmission circuit. Line 1 includes repeaters such as 1-1,, It 2,, preferably of the electron discharge type, which amplify telephonic waves propagated from west to east, that is from left to right. Line 2 includes repeaters such as P1,, 1%,, and R, for amplifying telephonic waves from right to left. Line 1 therefore transmits in one direction only, while line 2 transmits in the opposite direction only. At station A lines 1 and 2 are coupled to line 7, while at station 13 lines 1 and 2 are coupled to line 8. Lines 7 and 8 may terminate in central offices or in subscribers stations. The arrangements whereby the four-wire system is coupled to the terminating lines 7 and 8 are three-winding transformers, well known to the art. A balancing network or artificial line designed to have an impedance approximating that of line 7 is indicated at 9, and a balancing network for line 8 is indicated atlO. The broken portions of lines 1 and 2 indicate portions of considerable length and inay,;o-fcourse, include repeaters.

Frequently, under conditions experienced in practice, a considerable degree of unbalance obtains between the lines 7 and S and their respective artificial lines, with the result that currents transmitted through the repeating path in one direction are transmitted back to th'e sending end through the oppositely directed repeating path, and then transmitted again through the first repeating path, etc, so that both at thesending end and at the receiving end there are heard sounds lagging slightly behind the original voice vibrations and resembling echoes. The greater the time lag of the primary echo, the more disturbing the echoes are likely to prove.

Further, whenever points of either normal or transient impedance unbalance or discontinuity occur in a circuit, electrical energy is reflected from these points, and where, in circuits of thety'pe to which this invent-ion relates, the time required fortransmission is sufficiently great and the losses .imposed upon the reflected transmission are.

sufiiciently counteracted by the repeaters, the reflection currents may produce troublesome echo effects.

The permissible gain of the repeaters may be limited primarily by the disturbingef'fects of echo currents such as those referred to above, rather than by any singing of the re- 2 Therefore, even where balancing networks such as those indicatedin Fig. 1 cause the transmission losses around the repeater paths to be greater than the gains of the repeaters it is desirable, particularly when the time required for propagation of energy over the line is considerable, to substantially prevent normal transmission passing through either repeater path from returning to the sending end through the oppositely directed repeater path. In Figs. 1 and 2, means are shown for accomplishing this desirable result in such manner as to insure that the permissible gain of the repeaters will be limited only by their tendency to sing.

These means comprise two circuits such as the one shown in detail in Fig. 2. Each of these two circuits terminates at one end in leads 11 and 12 and at the other end inleads 13 and 14, and for convenience will be called an echo suppressor. One of the echo suppressors, the right hand one in Fig. 1,'has its leads 11 and 12 connected to line 1, preferably a the output side of repeater R and the other echo suppressor, the left hand one in Fig. 1, has its leads 11 and 12 connected to line 2 preferably at the output side of repeater R as indicated in Fig. 1. This left hand echo suppressor has its leads 13 and 14 connected to line 1, preferably substantially ator somewhat west of the juncture of line 1 with the leads 11 and 12 of the other echo suppressor, for example at the input side of repeater R, as shown, but by no- I means necessarily onthe input side of the repeater. The right hand echo suppressor has its leads 13 and 14 connected to line 2, preferably substantially at, or somewhat east of the juncture of line 2 with the leads 11 and 12 of theleft hand echo suppressor, for example at the input side of repeater R, as shown, but by no means necessarily on the input side of the repeater.

'- Each of the echo suppressors comprises a relay 15, which, when deenergiz'cd, closes its contact to connect leads 13 and 14 and thereby establish a short circuit across the line to which these leads are connected. The remaining parts of each echo suppressor are collectively designated K, as indicated by the block K in Fig. 2 and thetwo ressed on leads 11 and 12 from line '1 or 7 per second, the filter being preferably connected to leads 11 and 12 by means of a transformer 20 and an amplifier 21 of one or more stages and to the electron tube rectifier by means of a transformer 22. The tube 18 in turn feeds the relay 17 through a low-passfilter 23 having approximately the samefcut-ofi' frequency as filter 19. Were the relay 17 connected directly in the output circuit of the rectifier without filters 19 and 23, difiiculty might be encountered owing to'the fact that the cur rents of voice frequency would be amplified by the tube and the low frequency coinponents"particularly, or. the components.

having frequencies in the neighborhood of the mechanical resonance frequency of the relay tongue, would tend to cause chattering ofthe" relay during the periods when it should be held in the operating position by the rectified voice currents. possible to exclude these amplified voice currents from the relay 17 by merely inserting between the output of tube 18 and the relay 17 a low-pass filter, for instance, of

the general type shown in Fig. 7 of Camp" bell Patent NO.' 1,227,114, May 22, 1917, having a lower cut-off frequency than the filter 23. However, unless this cut-off point were made fairly low, the voice currents Whose frequencies were below the cut-off might cause chattering If the cut-off frequencywere made low, the speed of operation of the relay would be reduced, since the time required for-the propagation of the currents through a given numberof filter sections varies inversely as the cut-off frequency. Increased speed of operation It would be the high-pass filter 19 is used in the input circuit of the tube 18. When the high-pass filter is used, the only currentsreaching the rela 17 are those resulting from the detection of the higher voice frequencies, and he-ncethe relay will respond more exactly to the starting and stopping of the voice currents in leads 11 and 12. A still further advantage of this arrangement is that it prevents the relay from being operated by extraneous line currentsof low frequency such as cross-ringing, Morse thump and power interference, which are likely to be greater in-actual magnitude than the voice currents themselves.

The reason for employing the specific form of low-pass filter shown at 23 instead of the form shown in Fig. 7 of the Camp- 'bell Patent No. 1,227,114, referred to above,

is that under severe conditions of variable voice volume, lack of damping in the lowpass filter may cause chattering of the relay due to free oscillation set up on sudden changes in amplitude. When a resistancecondenser filter such as that shown in Fig.

2 is employed, both satisfactory filtering and sufiicient stability are attainable without necessitating the employing of inductance in the filter, and by the elimination of inductance the time required for the propagation of currents through the filter may be reduced.

Passing the voice energy through a highpass filter before rectifying and through a low-pass filter of approximately the same cut-off after rectifying. is the invention of R. V. L. Hartley. and is claimed in his aplication 'Serial No. 602,273, filed Novemer 20, 1922, entitled Transmitting electrical energy, assigned to the assignee of this application.

For reasons made apparent hereinafter, it is desirable that the relay 15 bequick operating as regards closure, but in effect, slow operating as regards opening of its contact. To obtain the effect of the slow operation without sacrificing quickness of closure, the circuit of relay 15 is controlled bya slow-acting auxiliary relay 24, which in turn is controlled by the lower contact of relay 17. In addition to its operating windin the relay 24 has a winding closed on itsel through a resistance25 which may be varied to adjust the time of release of the relay 24. When relay 17 opens its upper contact, relay 15 quickly places a short circuit across line 1 orline 2 through leads 13 and 14, and when relay 17 closes its lower contact relay 24 opens in the circuit of relay 15 a gap in scrice with the gap which the relay 17 has made at its upper contact. Thereafter, when relay 17 opens its lower contact and closes its upper contact, the circuit of relay 15 will not be completed until relay 24 releases. Preferably, the echo suppressors are located at a repeater station approximately midway between lines 7 and 8, and in that case the relays 24 in the two echo suppressors may be adjusted to have the same time lag.

In the operation of the system of Fig. 1, when currents, such for instance as voice currents, come from line 7 and line 1 into the repeater stat-ion comprising the echo suppressors and are amplified by repeater 3, the echo suppressor which has its leads 11 and 12 connected to the output of that repeater causes a short circuit through the armature of relay 15 to be established across line 2 at the input side of repeater 4. This subst-antially prevents any portion of the talking current from flowing back through repeat ers R R R etc-., from east to west due for instance to unbalance at station B or to reflection from station 13, or from any point east thereof; for the time required for operation of relay 15 is less than that required for propagation of currents from the output of repeater B, through station B to the input of repeater 1%,. Thus, echo currents flowing from station B to line 2 are not only prevented from circulating in the system, but are also prevented from reaching the echo suppressor which has its leads 11 and 12 connected to the output side of repeater R Therefore, the input of repeater R, can not be short circuited due to echo currents in line 2, even if the echo currents flowing in line 2 between station B and repeater R, are of the magnitude of the currents required inleads 11 and 12 to operate the. echo sup-- pressor. Any repeater connected in line 2 between station A and repeater R will prevent transmission of current from station A to repeater R, due to its unidirectional current transmitting property. The operation of the system in transmitting energy from east to west will not be described since it will be evident from the operation of the system in transmitting from line 7 to line 8 as just described.

With the system of Fig. 1 it is clear that if the echo suppressors be made sutficiently efiective in reducing the deleterious effects of echo-currents, the permissible gain of the repe-aters will no longer be limited by considerations of the magnitude of the echo currents while the system is in use (that is, while one or the other of the echo suppressors is in operation), but will rather be limited primarily by the requirement that for a satisfactory system the total transmission gains around the repeating paths should not exceed the total transmission losses around the repeating paths. In other words, the limit of the permissible repeater gains will be the singing point, and will be the singing point when the systeni is not in use (that is, when neither echo suppressor is in operation) rather than when the system is in use. It follows that, notwithstanding high repeater gains, it the repeating paths are long or the transmission losses around the repeating paths are large aside from the losses introduced at the threewinding transformers, satisfactory operation may be attained in many cases even though the degree of unbalance between the lines 7 and 8 and their balancing networks is large, or indeed even though the balancing networks are omitted as in a well known fourwirerepeating system disclosed in Van Kesteren Patent No. 1,189,411, Julyl, 1916.

The relay 24 makes possible rapidity of action in the rendering of an echo path-inoperative in spite of thenecessity for relatively slow action in the subsequent restoration of the path to operativeness, and is to insure that'when a party starts talking, over a portion of the circuit which transmits away from him, a portion of the circuit which transmits toward him is rendered inoperative soon enough to prevent him from hearing any considerable amount of talk which the-other party may have started before hearing the first party but after the first party started to talk. Thus, there is prevented any confusion which might otherwise arise in case a. party started to talk before hearing the distant party but after the distant party had started to talk. Moreover, confusion under such circumstances is avoided and yet, even after a cessation innormal transmission, circulation of echo currents due to that transmission is prevented.

If the repeater station at which the echo suppressors arev located is not midway between thelines 7 and 8, the relays 24 in the two echo suppressors-should be adjusted to different time lags since, in each echo suppressor, the'time elapsing between the operation of relay 17 and the consequent opening of relay should not be. less thanthe time required for propagation of-currents from the repeater station at which the echo suppressor is located to a terminating setof the four-wire circuit and back to theleads 13 and 14 of the echo suppressor. If the time elapsing is too great, the ability of the listener to break in on the talker may be unduly limited.

By connecting leads 11 and 12 of the echo suppressors to the outputs of the repeaters, the echo suppressors receive more voice current energy than they would receive were the leads lland 12 connected'tothe repeater inputs.

Although, as noted above, in the system shown in Figs. 1 and 2 the limit of the permissible gain of the repeaters is the singing point when the system is not in use, such a system can have its repeaters set for gains ust Wltllln a working margin of the singing point when the, system is not in use and can be arranged so that when normaltrans.

iently be arranged to thus increase the transmitting etliciency of the repeating path through which the desired transmission isto pass, Fig. 3 shows a systemwhich may be considered a modification of the system of Fig. 1 in that a different form of echo sup-i pressor is employed and moreover, in that the system of Fig. 3. embodies means for causing the desired increase in the transmitting efficiency of the operative repeating path. This feature is. of special utility in cases where good balance between the lines 7 and 8 and their balancing networks can only be attained with unwarranted expense, due for instance toimpedance irregularities in the lines, and is also especially useful in cases in-which it is desirable to omit the balancing networks. I l V In Fig. 3, lines 1, 2, 7 and 8, the balancing networks 9 and 10 and the repeaters 11 ,3 R and R correspond to the lines, networks and repeaters so designated in Fig. 1; and repeaters R' and B correspond to the repeaters R and R, m Fig. 1 exceptthat the potentiometers for the repeaters R and R',, are shown separate from the repeaters and are designated and 26respectively. A relay 27 responsive to voice currents to,

in'efi'ect, open its contact, is connected to the output of repeater RQ. This voice relay may be, for instance, ofthe vibrating type disclosed in S. Gr. Brow'n, Patent No. 1,185,472,. May 30, 1916.,

A similar relay 28 is connected across the output of repeater R',,. The voice relay 27 controls a. direct current relay 29 and the voice relay 28 controls a direct current relay 30. The relay 29 controls the potentiometer 25 and an echo reducing and gain reducing shunt circuit across the input of repeater R,, the shunt circuit containing a variable resistance 31. Similarly the relay 30 controls the potentiometer 25' and an echo reducing and gain adjusting shunt circuit, containing a variable resistance 32, across the input of repeater R When no trans mission is taking place in the system, the shunts comprising resistancefil and 32 are open circuited and the potentiometers 2 5 and 26 are set at some point sufficiently below the maximum gain setting to prevent singing of the repeaters due tonoise currents on the line. y r

In the operation of the system, assuming ieo loo

hpeater R due to the change of its potentiometerconnection is made such as to not exceed the gain reduction im osed upon repeater R; by theclosing'of t e shunt circuit comprising resistance 31, and therefore the repeaters still willfn'ot *sing, 'although the gem of repeater R, has been increased to a Jvalue larger than was originally permissible. The resistanceBl isof course adjustedto a valuesufiiciently low to reduce the oirculation of echo currents to the desired degree.

xAny repeater-in line Q between repeater- R and stationfaB wil'lprevent transmission of.

a current from station E to repeaterRQdue to s its unidirectional current transmitting property. The operation of the system in transmitting from line 7 to line 8 is similar "tothat in'trans'mitting from line 8toline 7, and needs no separate description.

, In changing from thev neutral condition of the circuit to the operating condition in either direction, or in changing from "either operating co-ndition to the neutral condition ofthe circuit, changes" made on the repeater which is to haveits gain reduced and'the changes made on the other repeater should occur in suchisequence as to always maintain "thQCII'CIIIt 1n the non-singing OOIlClltlOIl'.

It is noted that this type of circuit avoids icircuit. interruptions due to operation of relay contacts actually connected in the transfmission circuit (such interruptions as would occur, for instance,'ifthe gain decreases of the repeaters Were accom hshed byopening 1 their-input circuits),- an that transmission in" either "direction over the circuit .need never abeaentirely p evented although telephonecurrents wi l "be. attenuated more at V "onetime "than at another. If there are telej pl'1one=currents passing over the circuit,-at any time, "ofgsuflicient magnitude to be heard rat the receiving station, ,no part of these currents will be lost due to switching foper'ations in the repeater circuit. a More- .over no elijcks due to changes of D. C. space current in the' amplifiers are sent'o'ut on the "lines asw ouldbe the'case were'the' gain decreases of the repeaters produced by varying their filament curren'ts or their D. 0. plate voltages or their D. G. grid potentials. Y

Ithas'been indicated above that in the system of Figsjl and 2, and also in the system' of Fig-1'3, the balancing networks 9 and 10 may beomitted, provided the transmission gains around the repeating V 7 never greater than the transmlsslon losses rection.

paths are around these paths. As representing such a system minus the balancing networks, Fig. 4: s 1OWS a system which may be considered a modification of the system of Figs. 1 and 2 in that, as in Fig. 3, a different form of echo suppressor isemployedand means is "provided such that when normal transmission passes through one or the other of the repeater paths the gain of the repeating means in that path is increased, and moreover in that the networks 9 and 10 shown'in Figs. 1 and 3 areomitted. Fig. 4 is like Fig. 3 except for the omission of these networks, and hence need not be further described.

The general principles herein disclosed ma be embodied in many organizations wi ely different from those illustrated withou't'departing from the spirit of the invention as defined in the appended claims.

Although the invention has been set forth with especial reference to its application to the repeatingof voice frequency currents, it

is of course also applicable to the transmisslon and repeating of currents of otherfrequencies, whether hlgher or lower, as for in- "stance currents of the frequencies commonly used for radio and carrier currents.

f The invention claimedisz Y i 1. The method of operating a twoway repeating system including two two-wire circuits terminating at points geographicall remote from each other andtwo opposltely directed repeatered paths joining said terminations, which comprises maintaining 'saidpaths operatively connected to transmit between said two-wire circuits in the absence of transmission through the system, and during transmission in the system, maintaining one of said paths at less the absence of transmisison, for repeating in opposite directions between said two-wire circuits, and two means, each responsive to transmission in onl one direction over the system for substantially preventing the passage of current through either the other of sad two means or the path in the other di- 3. A two-way signalling system comprising paths adapted to transmit in opposite directions between two points, and a means {connected to each path at apoint remote from the ends of'said path and responsive to transmission in the connected path for path and the other theassociated path for substantially preventing the passage of current through the other path and through the other .of said two means. I

. -.5. A two-way signalling system comprisingjtwo two-wire circuits, paths operative to transinit in opposite directionsbetween said circuitsin the absence of transmission in saidvsystem, the. net transmission loss around the loopcomprising said paths'be .ing then greaterthan zero, a switch at a point in each ofsaid paths for, controlling the transmitting efficiency of the path, and

a voice operated relay connected to eachfof said paths and adaptedwhen energized by voice ,cnrrentstocontrol the switch in the other path, each of said relays-beingconnected to its associated pathatsucha point, relative to the switchpfor thatpath, that when the switch for that-pathis. operated by the other relay, current is substantially prevented from passing through the relay connected to thatpath.

6. A ..two -way signalling system; c ompris ing two two-wire circuits, paths operative to transmit in opposite directions between said circuits inthe absence of transmission in said system, means tending to prevent circulating currents from each ofsaid paths from flowingin the other'path,aswitch at a pointin each of said paths for controlling the transmitting efliciencyof the path, and

a voice operated relayconnectedto each of said paths and adapted when energized by voice currents to. control the switch in the other path, each of said relays being connected toits associated path at such a point,

relative. to the switch, for that path, that when. the switch for that pathv is operated by the other relay, current is subs'tantially prevented frompassingthrough the relay connected to that path.

i 7. In combination, two line sections, two unidirectional oppositely directedpaths between said line sections,-impeda nce elements substantially balancing the impedances of said line sections respectively, said unidirectional paths being so related to said; line sections and to said balancing impedanQes that only a relatively small amount of cur-.

rent traversing one of said paths is fed back :to the other of said paths,.and a \means'associated with each path and operable by voice currents in that, path for reducing the transmitting efliciency ofthe otherjof said .said first mentioned means. i

.naaasso paths and rendering the other means inoperallillej by said currentfed back to said other pat i 8. In a circuit haviii'g"repeatered paths normallyin condition. to transmit signals in difi'erent directions,- means-tending to prevent circulating currents-fromeach of said paths from flowingjn the other of said paths and a means associated with each path and operable by signal currents inthe other path for cutting off transmission through the associated path at such I a point as --to prevent operation of the corresponding means associated .--with,;the by said circulating currents.

9. A two-waysignalling system comprisingtwo two-wire circuits two paths which include amplifying means ,and are ,conditioned to transmit 11'1- oppos te directions between. said circuits in the absence of transmission inthesystennthe total transmission loss around the loop, comprising: said paths beinggreater'than thetotal gain of said amplifying means, and .means associated with eachaof said pathsand. responsive to transmissionin the associated path :for re- ;ducing the transmission.efliciencyof the other path and for preventing Rtlle other means, from being operated.

105A twoeway s gnalling system comprising twosline SGCtlOIlS, two. transmission paths, connecting. said line w sections, said paths being normally conditioned to ;transmit in opposite directions, the net transmission loss around the loopcomprising said paths beingratleast as great as .zero, means operablejby signalling currents in .one of said paths for reducing the transmission efficiency of the other of .said paths,and

means operable by signalling currents, in 7 said otherpath'to prevent the operation of 11. A two-way signalling system ccomprising two line sections, two transmission paths connecting said line. sections, said paths being normally conditioned :to transparatus in said paths, the total transmission o-ss aroundthe loop comprising said paths bein greater. than the total-.gailiyof said amplifying apparatus, 'means, operableqby signalling currents. in one directionover the system for reducing, thetransmission efficiency of the path-'in-the; other direction,

7 and means operable by signalling currents in said other direction to prevent.,,op eration of sa d first mentioned means. 1; i

t 12. A two-way signalling] systenr coinprising two paths. normally conditioned to transmit in opposite directions-between ,two points, the net transmission loss around the loop comprising said pathsbeing at least as great as zero,' means operable by signalling currents in one direction for reducing the transmissioneificiency of thes -path in the mit in oppositedirections, amplifying apother direction, and means operable by signalling currents in said other direction to prevent the operation of said first mentioned means. .13. A two-way signalling system comprising two line sections, two transmission paths connecting said line sections, said 1 paths being normally conditioned to transmit in opposite directions, amplifying apparatus in said paths, the total transmission o'ss around the loop comprising said paths being greater than the total gain of said amplifying apparatus, means operable by voice currents in one of said paths for reducing the transmitting efiiciency of the other of said paths, and means operable by voice currents in said other path to prevent operation of said first mentioned means.

14. A a two-way signalling system comprising. two line sections, two transmission paths connecting said line sections, said paths being normally conditioned to transmit in oppositedirections, amplifying apparatus "in said; paths, the total transmission loss .aroundthe loop comprising said paths being greater than the total gain of said amplifying apparatus and being substantially equal to the sum of the transmission losses in theindividual paths, means operable by voice currents in one of said paths for reducing the transmission eificiency of the other of said paths, and means operable by voice currents in said other path to prevent operation a of said first mentioned means.

15. A twoway' signalling system comprising paths which include amplifying means andwhich are adapted to transmit in opposite directions, apparatus tending to prevent circulating currents from one of 16. A two-way signalling system comprisin two two-wire circuits, two paths operative to transmit in opposite directions between said circuits in the absence of transmission in said system, and means associated with said paths and responsive to a given transmission in said system for altering the tranmitting efficiency of each of said paths.

17. The method of operating a two-way repeater circuit including oppositely directed repeating paths operatively connected to transmit between two lines in the absence of transmission in the circuit, which comprises maintaining the transmission gains in said circuit at such valves in the absence of normal transmission in the circuit, that sin ing of the repeaters can not take place, an whenever normal transmission in the circuit begins, increasing the transmission efficiency of the circuit in one direction.

18. The method of operating a two-way repeater system including two two-wire circuits and two oppositely directed repeatered paths connecting said two-wire circuits, which comprises maintaining thetransmission gains in said paths sufliciently low to prevent singin of the repeaters in the absence of norma transmission in the system, and, whenever normal transmission in the system begins, increasing the transmitting efliciency of one of said paths and decreasing the transmitting efliciency of the other path sufiiciently to prevent singing of the re eaters.

19. n a signalling system, a sending station, an outgoing path for signals, a return path for a portion of said outgoing path, said paths being so connected as to permit circulating currents in said return path,and signal responsive means for simultaneously increasing the transmitting efficiency of one of said paths and decreasing the transmitting efficiency of said other path.

In witness whereof, I hereunto subscribe my name this 30th day of January, A. D.

EDGAR D. JOHNSON. 

